How a switch handles a broadcast.

"Recall that SC0 is the management interface used in Catalyst switches such as the 4000s, 5000s and 6000s.

This is where the management IP address is assigned to a Catalyst Supervisor.

Because the CPU processes all broadcast packets (and some multicast packets) received on this interface, it is important to not overwhelm the CPU."

"Never mix end-user traffic with control and management traffic.

When implementing this principle, you must generally choose one of two designs:

- Use VLAN 1 for all control and management traffic while placing end-user traffic in other VLANs (VLANs 2–1000).

- Use VLAN 1 for control traffic, another VLAN (such as VLAN 2) for management traffic, and the remaining VLAN for end-user traffic (such as VLAN 3–1000)."

It seems that a broadcast is handled in different ways if it belongs to different VLANs. This principle is reasonable. But I have seen different resources where an output line of the command show mac-address-table is:

All ffff.ffff.ffff STATIC CPU

It seems that a broadcast is handled by the CPU for all VLANs and this is not reasonable.

I do not know exactly which models those output lines belong to, but I think that principles are not changed since 4000s, 5000s and 6000s. May be the output line remember the switch that broadcasts are not handled as unicasts, and after that only broadcasts

in particular VLANs are sent to the CPU. May be that only because broadcasts must be flooded, this process requests a CPU intervention but it is not reasonable. It should be reasonable that after the decision of VLAN management traffic, only broadcasts in VLAN 1 and in VLAN management if different are sent to the CPU, broadcasts in VLANs user traffic should be switched by the switch fabric. What do you think about?

Re: How a switch handles a broadcast.

When one transmitter needs to reach all of the receivers in the network, it sends a broadcast. Broadcast frames are received by every host on the segment. The broadcast domain at Layer 2 is referred to as the MAC broadcast domain. The MAC broadcast domain consists of all devices on the LAN that receive broadcast frames from a host to all other machines on the LAN.

A switch is a Layer 2 device. When a switch receives a broadcast, it forwards it to each port on the switch except the incoming port. Each attached device must process the broadcast frame.

When two switches are connected, the size of the broadcast domain is increased.

The overall result is a reduction in available bandwidth as all devices in the broadcast domain must receive and process the broadcast frame.

With today’s day and age, connectivity has become ubiquitous. Not only do people expect a reliable and fast connectivity in their offices, but also at their favorite coffee shops and grocery stores. Setting up the network to provide such connectivity is n...
view more

Join us November 15 at 10 am PT as we talk to Cisco’s senior Enterprise Networks executives about our recent announcements around SD-WAN and the Catalyst expansion.
As always, our Cisco experts are ready to answer your questions, so...
view more

The more I learn about the applications Cisco partners are building using the DNA Center’s open APIs, the more I am impressed by the creativity by which they are using these APIs. Their solutions are helping network operations by removing the guesswork an...
view more

The Catalyst 9000 family of switches is at the heart of Cisco’s Network Intuitive strategy. The success of Catalyst 9000 Series Switches is inspiring! We are extending the success further by introducing our newest Cisco Catalyst 9200 Series Switches...
view more